The present invention relates to a method for controlling a ring network, and more particularly to a method for controlling stations in response to occurrence of a failure in a communication network system including stations connected by a double-ring transmission path.
In a ring or loop network system in which a plurality of stations are coupled by a pair of ring transmission lines having opposite directions of signal transmission, data is normally transmitted among the stations through one of the transmission lines and the other transmission line is used for backup purposes. If a failure occurs in the transmission line or one of the stations in such a ring network, the transmission line is switched to the backup line, or the signal is returned from one transmission line to the other transmission line between two stations adjacent to a trouble location so that the data is transmitted by bypassing the trouble location.
In a system shown in "Design and Performance of 100 Mb/s Optical Accessing Loop Network", Nippon Communication Association Technical Report, Vol 84, No. 214, SE84-102 (Nov. 30, 1984), pages 31-36, in order to reconfigure the transmission line such as switching of the transmission line or formation of a loop-back path, the network is constructed to include one center station (CS) and a plurality of remote stations (RS), and when an error is detected in the transmission line, the CS instructs the loop-back to all RS's. The loop-back instruction frame is simultaneously sent to the two transmission lines. When the RS receives the instruction, it relays the frame to the next station on the same transmission line as the incoming line and also forms a loop-back path from that transmission line to the other transmission line to send out the same frame to the other transmission line. When the RS receives the loop-back instruction frames from both transmission lines, it releases the loop-back path. As a result, only the two RS's adjacent to the error location hold the loopback paths. Another system in which the center station CS is provided to control the network configuration is disclosed in Japanese Unexamined Patent Publication No. 59-154843. In such systems, if an error occurs in the center station CS, the control of the network configuration is disabled. Therefore, a backup for the center station CS has to be provided.
On the other hand, the article, "A highly reliable distributed loop network architecture" by A. Grnaroy, L. Kleinrock and M. Gerla, disclosed at the 10th International Symposium on Fault Tolerant Computing, Oct. 1-3, 1980, proposes a system in which a center station CS is not provided but all stations ST in the network have an equal role so that the reconfiguration of the network is equally and distributedly controlled by all stations ST. In this system, the backup of the center station is not necessary. However, the following point should be taken into account. When an error occurs, it destroys its own communication function. the time required to check for errors differs from station to station depending on the presence or absence of a problem and the degree of the problem thereof and the number of communication means (e.g. external interfaces) involved. Since the stations ST are connected in series in the ring network, the counter measures for the error (e.g. reconfiguration of the ring) must be effected with all stations ST being is synchronism. That is, when the network performs an overall operation (e.g. reconfiguration), all stations ST must be in synchronism with the operation. If any one of the stations ST is not in synchronism, it appears as an error to other stations ST.